The subject of the present invention is a tool for measuring a torque such as, for example, an electronic dynamometer wrench, that makes it possible to ascertain the value of a torque exerted on a tightening member (nut, screw, bolt or the like) rotated by means of this wrench, and consequently to monitor the tightening accomplished with this wrench.
One of the problems currently encountered in devices for measuring tightening (or untightening) torque of the dynamometer wrench type is that of error due to uncertainty over the point of application of the operating force on the handle of the wrench. In practice, the user does not always position his hand exactly at the same location and/or does not distribute the force between his various fingers in a constant manner.
The version generally regarded as the simplest is a dynamometer wrench 1 using resistive extensometers and electronic signal processing, and is represented in FIG. 1. It comprises an operating head (of a variable model) for the component to be screwed, a flexible part 4 (handle) equipped with an extensometer (M) serving to measure the force F applied perpendicularly to the longitudinal axis of the handle 4, and a manual grip 3 serving to apply the force F at a variable point P. The geometries and embodiments of these various components can, of course, be very variable. The head for operating the component to be screwed can, for example, include a fork wrench or a socket-drive square 5 or a "universal" adaptor.
This simple solution uses a measurement of the bending of the handle of the wrench or of an intermediate component representative of the bending of the handle. The means for measuring bending are required to have a non-zero distance from the axis of the head of the wrench. The coefficient between the measured value and the torque (Co) transmitted by the wrench depends on the position of the point of application of the operating force. Its value is: EQU Co=F.times.L (1)
The measured value being: EQU Me=F.times.(L-D) (2)
Direct calculation of the torque from the measured value would therefore lead to an error Er of value equal to: EQU Er=-F.times.D (3)
It is noted that the absolute value of this error depends only on the value of the force F and is independent of the position of the point of application of the force.
The coefficient is therefore EQU L/(L-D) (4)
And the exact value of the torque can be calculated (to within a coefficient) with the aid of the formula: EQU C=mex(L/(L-D)) (5)
Equation (5) demonstrates that the value obtained corresponds exactly to that of the torque applied to the component to be screwed only if the position of the point of application P of the operating force F is constant. For practical applications this imprecision limits the possibility of using this simple solution when seeking precise measurements. Also, a number of embodiments have been proposed for producing dynamometer wrenches not exhibiting this defect.
A first embodiment, described in particular in French Patent 2,400,996, consists of placing the means for measuring the effort in manner which is physically or functionally concentric with the axis of the screw or nut tightened (untightened) by the wrench. This device leads to an increase in the volume of the wrench in the neighborhood of its head, thus posing problems of accessibility in numerous cases. Moreover, certain types of drives, fork wrench in particular, are not compatible with this solution.
Another entirely mechanical embodiment (French Patent 1,034,502) consists of producing, by means of two blades converging towards the axis of the head of the wrench, a structure which deforms preferentially under the effect of a torque. The influence on the measuring elements eof the forces other than the torque to be measured is markedly reduced, and the measurement can be regarded as depending solely on this torque. The association of this device with electronic measurement by means of resistive extensometers ("strain gauges") is described in French Patent 2,584,330.
A third embodiment, described in U.S. Pat. No. 4,006,629, consists of providing two independent measuring devices located different distances D1 and D2 from the axis of the head of the wrench. The ratio of the values measured by the two devices is influenced by the position of the point of application of the force and makes it possible to thereby determine this position. Once the latter is known, the exact ratio of the measured value to the torque can be determined and the latter can therefore be calculated exactly. In practice, simple addition of the measured values M1 and M2 with suitable coefficients allows an overall solution, and therefore dispenses with carrying out explicit calculations. By reason of the very principle of this device, the overall signal provided by the elongation sensors has a markedly smaller value than the signal corresponding to simple bending, and is therefore more sensitive to disturbances.
A fourth embodiment, described inter alia in French Patent 2,538,741, consists of mechanically coupling the metallic component whose deformation is measured and the handle of the wrench in such a way that only the forces corresponding to the transmission of a torque are transmitted to the part serving in the measurement of the force. In general, this type of solution employs mechanical devices of the articulation type which, by reason of the imperfections inherent in this function, leads to a limitation on the possible precision.
These various solutions lead to markedly more complex and more expensive embodiments than the straight-forward measuring of bending in the neighborhood of the handle of the wrench.